# std::pmr::polymorphic_allocator<T>::construct

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< cpp‎ | memory‎ | polymorphic allocator

template< class U, class... Args > void construct( U* p, Args&&... args ); | (1) | (since C++17)
template< class T1, class T2, class... Args1, class... Args2 > void construct( std::pair<T1, T2>* p, std::piecewise_construct_t, std::tuple<Args1...> x, std::tuple<Args2...> y ); | (2) | (since C++17) (until C++20)
template< class T1, class T2 > void construct( std::pair<T1, T2>* p ); | (3) | (since C++17) (until C++20)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, U&& x, V&& y ); | (4) | (since C++17) (until C++20)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy ); | (5) | (since C++17) (until C++20)
template< class T1, class T2, class U, class V > void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy ); | (6) | (since C++17) (until C++20)
template< class T1, class T2, class NonPair > void construct( std::pair<T1, T2>* p, NonPair&& non_pair ); | (7) | (since C++17) (until C++20)

Constructs an object in allocated, but not initialized storage pointed to by p the provided constructor arguments. If the object is of type that itself uses allocators, or if it is std::pair, passes *this down to the constructed object.

1) Creates an object of the given type `U` by means of uses-allocator construction at the uninitialized memory location indicated by p, using *this as the allocator. This overload participates in overload resolution only if `U` is not a specialization of std::pair.(until C++20)

2) First, if either `T1` or `T2` is allocator-aware, modifies the tuples x and y to include `this->resource()`, resulting in the two new tuples `xprime` and `yprime`, according to the following three rules: 2a) if `T1` is not allocator-aware (std::uses_allocator<T1, polymorphic_allocator>::value==false) and std::is_constructible<T1, Args1...>::value==true, then `xprime` is x, unmodified. 2b) if `T1` is allocator-aware (std::uses_allocator<T1, polymorphic_allocator>::value==true), and its constructor takes an allocator tag (std::is_constructible<T1, std::allocator_arg_t, polymorphic_allocator, Args1...>::value==true, then `xprime` is std::tuple_cat(std::make_tuple(std::allocator_arg, *this), std::move(x)). 2c) if `T1` is allocator-aware (std::uses_allocator<T1, polymorphic_allocator>::value==true), and its constructor takes the allocator as the last argument (std::is_constructible<T1, Args1..., polymorphic_allocator>::value==true), then `xprime` is std::tuple_cat(std::move(x), std::make_tuple(*this)). 2d) Otherwise, the program is ill-formed. Same rules apply to `T2` and the replacement of y with `yprime`. Once `xprime` and `yprime` are constructed, constructs the pair p in allocated storage as if by ::new((void *) p) pair<T1, T2>(std::piecewise_construct, std::move(xprime), std::move(yprime));. 3) Equivalent to construct(p, std::piecewise_construct, std::tuple<>(), std::tuple<>()), that is, passes the memory resource on to the pair's member types if they accept them. 4) Equivalent to
    construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(x)),
                                           std::forward_as_tuple(std::forward<V>(y)))
5) Equivalent to
    construct(p, std::piecewise_construct, std::forward_as_tuple(xy.first),
                                           std::forward_as_tuple(xy.second))
6) Equivalent to
    construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(xy.first)),
                                           std::forward_as_tuple(std::forward<V>(xy.second)))
7) This overload participates in overload resolution only if given the exposition-only function template
    template< class A, class B >
    void /*deduce-as-pair*/( const std::pair<A, B>& );
, /*deduce-as-pair*/(non_pair) is ill-formed when considered as an unevaluated operand. Equivalent to
    construct<T1, T2, T1, T2>(p, std::forward<NonPair>(non_pair));
| (until C++20)

### Parameters

p | - | pointer to allocated, but not initialized storage
args... | - | the constructor arguments to pass to the constructor of `T`
x | - | the constructor arguments to pass to the constructor of `T1`
y | - | the constructor arguments to pass to the constructor of `T2`
xy | - | the pair whose two members are the constructor arguments for `T1` and `T2`
non_pair | - | non-`pair` argument to convert to `pair` for further construction

### Return value

(none)

### Notes

This function is called (through std::allocator_traits) by any allocator-aware object, such as std::pmr::vector (or another std::vector that was given a `std::pmr::polymorphic_allocator` as the allocator to use).

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2969 | C++17 | uses-allocator construction passed `resource()` | passes *this
LWG 2975 | C++17 | first overload is mistakenly used for pair construction in some cases | constrained to not accept pairs
LWG 3525 | C++17 | no overload could handle non-`pair` types convertible to `pair` | reconstructing overload added

### See also

construct [static] | constructs an object in the allocated storage (function template)
construct (until C++20) | constructs an object in allocated storage (public member function of `std::allocator<T>`)

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